The present invention relates to rotary electric motors, more particularly to motors having a plurality of rotor elements and stator elements comprising a plurality of poles that are aligned in a direction parallel to the axis of rotation of the motor.
The progressive improvement of electronic systems, such as microcontroller and microprocessor based applications for the control of motors, as well as the availability of improved portable power sources, has made the development of efficient electric motor drives for vehicles, as a viable alternative to combustion engines, a compelling challenge. Electronically controlled pulsed energization of windings of motors offers the prospect of more flexible management of motor characteristics. By control of pulse width, duty cycle, and switched application of a battery source to appropriate stator windings, functional versatility that is virtually indistinguishable from alternating current synchronous motor operation can be achieved. The use of permanent magnets in conjunction with such windings is advantageous in limiting current consumption.
The above-identified copending related U.S. patent application of Maslov et al., Ser. No. 09/826,423, identifies and addresses the need for an improved motor amenable to simplified manufacture and capable of efficient and flexible operating characteristics. In a vehicle drive environment, it is highly desirable to attain smooth operation over a wide speed range, while maintaining a high torque output capability at minimum power consumption. Such a vehicle motor drive should advantageously provide ready accessibility to the various structural components for replacement of parts at a minimum of inconvenience. The copending related U.S. application incorporates electromagnet poles as isolated magnetically permeable structures configured in an annular ring, relatively thin in the radial direction, to provide advantageous effects. With this arrangement, flux can be concentrated, with virtually no loss or deleterious transformer interference effects in the electromagnet cores, as compared with prior art embodiments. While improvements in torque characteristics and efficiency are attainable with the structure of the identified copending application, further improvements remain desirable.
To this end, the above-identified copending related U.S. patent application of Maslov et al., Ser. No. 09/826,423, seeks to optimize rotor parameters such as the grade of the magnet, the energy density and the overall magnetic characteristics of the magnet grade, the size and the dimensions of the magnet that can adjust the effective working permeance and the overall operating condition of the magnet when it is part of the rotor, the temperature stability of the magnet, the finishing, coating and post processing steps taken in manufacturing of the magnets for the intended application, the stability of the magnetization over the curvilinear surface of the magnet, uniformity of the radial polarization of the magnet, the adjacent gap between two separate magnets, the mechanical features of the edges of the magnets, and the return flux path of the magnet as provided by a back iron ring section.
The Maslov et al. applications recognize that isolation of the electromagnet groups permits individual concentration of flux in the magnetic cores of the groups, with virtually no flux loss or deleterious transformer interference effects with other electromagnet members. Operational advantages can be gained by configuring a single pole pair as an isolated electromagnet group. Magnetic path isolation of the individual pole pair from other pole groups eliminates a flux transformer effect on an adjacent group when the energization of the pole pair windings is switched. The lack of additional poles within the group eliminates avoids any such effects within a group.
While significant advancements have been derived from the above described efforts, it has been found that potential benefits to be gained from utilization of three dimensional aspects of motor structure have not been fully realized. In the motors of the copending applications, all stator and rotor poles are circumferentially located about the rotational axis and are coextensive in the axial direction. For a given air gap diameter, the total number of poles, twice the number of pole pairs, is limited by practical physical capabilities. The optimum active surface area of the individual poles that produce flux interaction between rotor and stator is thereby determined, as well as the number of poles that produce such interaction. A structural configuration in which flux distribution is more highly concentrated, while providing a greater number of poles with the same individual active air gap surface areas and/or greater total active air gap surface area for a motor with the same air gap diameter would be highly desirable to obtain even greater performance.
The present invention furthers the above-described needs of the prior art and provides additional advantages for configurations such as the isolated individual pole pair arrangements disclosed in the above identified Maslov et al. applications.
Advantages of the present invention are achieved, at least in part, by extending the radial flux distribution interaction between stator and rotor poles along the axial direction of the motor.
Additional advantages of the present invention can be realized from the use of materials by which a soft magnetically permeable medium is amenable to formation of a variety of particular shapes. For example, core material may be manufactured from soft magnet grades of Fe, SiFe, SiFeCo, SiFeP powder material, each of which has a unique power loss, permeability and saturation level. Core geometries and core dimensions of stator elements, with relevant tolerances, can be formed without the need to form laminations and thus optimize the magnetic potential gradient developed between coupled poles of rotor permanent magnets and stator electromagnets.
The aforementioned advantages are manifested in structural features of the invention, at least in part, wherein the motor comprises a rotor and stator each disposed in an angular ring configuration and spaced from each other by an annular air gap. The stator comprises a plurality of magnetically permeable core segments with coils wound thereon, the core segments being separated from direct contact with each other and disposed along the radial air gap. Each stator segment comprises a pair of poles aligned with each other in a direction parallel to the axis of rotation. The stator thus has two sets of axially displaced poles, the poles of each set being in axial alignment. Each core segment pole pair thus comprises a pole of one of the sets structurally connected with a corresponding pole of the other set. In this arrangement, the total number of pole pairs is equal in number to the number of poles in a set. Preferably, the poles of each stator core segment is joined by a linking portion. The stator winding may be formed on the linking portion, whereby application of current to the winding effects opposite magnetic polarities in the pole pairs. Alternatively, the winding may comprise two sets of coils wound respectively in opposite directions to each other on corresponding poles of the stator pole pair and which may be connected in series or parallel, whereby application of current to the connected coil sets effects magnetization of the poles of the stator core segment in opposite magnetic polarity. In either alternative, reversal of the current applied to the winding will result in reversal of the magnetic polarity of the stator poles.
The rotor preferably comprises a plurality of permanent magnet elements successively alternating in magnetic polarity along its inner annular surface. Each rotor element comprises a pair of poles aligned with each other at the air gap in a direction parallel to the axis of rotation, the poles having opposite magnetic polarities. Thus, the rotor also has two sets of axially displaced poles with the poles of each set in axial alignment. The sets of rotor poles preferably are axially coextensive, and in axial alignment, with corresponding stator pole sets.
Another advantage of the present invention can be attributed to rotor structure that is configured to enhance the direction of magnetic flux in the radial direction across the air gap. For example, each rotor pole may be magnetized to exhibit one magnetic polarity at the air gap at its inner pole surface and an opposite magnetic polarity at its outer pole surface to concentrate the magnetic flux in the radial direction. The rotor elements may be contiguous to each other and mounted on a ferromagnetic annular back plate. Preferably, the rotor elements are not in direct contact with each other to thereby enhance isolation of the magnetic paths. Alternatively, each rotor magnetic element may further comprise a magnetically permeable section extending along the axial direction as a magnetic back plate, each pole of the rotor element mounted at a respective axial end of the back plate. In this arrangement, a separate back plate portion is provided for each rotor element, rather than a continuous magnetic annular back plate, thereby to avoid stray flux distribution. In either embodiment, a non-ferromagnetic annular back plate is provided to surround the rotor elements.
In yet another preferred embodiment, each rotor permanent magnetic element further comprises an additional magnet portion between the pole portions of the element, the additional magnetic portion directing flux between the pole portions of the element in the axial direction, thus further improving the active flux distribution pattern.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.